The invention relates to a method of treating an anti-reflective coating on a substrate and also to a method of forming an electronic device. The method of forming an electronic device includes treating an anti-reflective coating on a substrate.
Microelectronic devices used in the construction of integrated circuits are manufactured by means of photolithographic techniques. Fabricating various structures, particularly electronic device structures, typically involves depositing at least one layer of a photosensitive material, typically known as a photoresist material, on a substrate. The photoresist material may then be patterned by exposing it to radiation of a certain wavelength to alter characteristics of the photoresist material. Typically, the radiation is from the ultraviolet range of wavelengths. The radiation preferably causes desired photochemical reactions to occur within the photoresist. Preferably, the photochemical reactions alter the solubility characteristics of the photoresist, thereby allowing removal of certain portions of the photoresist. Selectively removing certain parts of the photoresist allows for the protection of certain areas of the substrate while exposing other areas. The remaining portions of the photoresist may be used as a mask or stencil for processing the underlying substrate.
An example of such a process is in the fabrication of semiconductor devices wherein, for example, layers are formed on a semiconductor substrate. Certain portions of the layers may be removed to form openings through the layers. The openings may allow diffusion of desired impurities through the openings into the semiconductor substrate. Other processes are known for forming devices on a substrate.
Devices such as those described above, may be formed by introduction of a suitable impurity into a wafer of a semiconductor to form suitably doped regions therein. In order to provide distinct P or N regions, which are necessary for the proper operation of the device, introduction of impurities should occur through only a limited portion of the substrate. Usually, this is accomplished by masking the substrate with a diffusion resistant material,which is formed into a protective mask to prevent diffusion through selected areas of the substrate.
The mask in such a procedure is typically provided by forming a layer of material over the semiconductor substrate and, afterward creating a series of openings through the layer to allow the introduction of the impurities directly into the underlying surface within a limited area. These openings in the mask are readily created by coating the mask with a material known as a photoresist. Photoresists can be negative photoresist or positive photoresist materials.
A negative photoresist material is one which is capable of polymerizing and being rendered insoluble upon exposure to radiation. Accordingly, when employing a negative photoresist material, the photoresist is selectively exposed to radiation, causing polymerization to occur above those regions of the substrate which are intended to be protected during a subsequent operation. The unexposed portions of the photoresist are removed by a solvent which is inert to the polymerized portion of the photoresist. Such a solvent may be an aqueous solvent solution.
Positive photoresist material is a material that, upon exposure to radiation, is capable of being rendered soluble in a solvent in which the unexposed resist is not soluble. Accordingly, when applying a positive photoresist material the photoresist is selectively exposed to radiation, causing the reaction to occur above those portions of the substrate which are not intended to be protected during the subsequent processing period. The exposed portions of the photoresist are removed by a solvent which is not capable of dissolving the exposed portion of the resist. Such a solvent may be an aqueous solvent solution.
Photoresist materials may be similarly be used to define other regions of electronic devices.
In an effort to increase the capability of electronic devices, the number of circuit features included on, for example, a semiconductor chip, has greatly increased. When using a process such as that described above for forming devices on, for instance, a semiconductor substrate, increasing the capability and, therefore, the number of devices on a substrate requires reducing the size of the devices or circuit features. One way in which the size of the circuit features created on the substrate has been reduced is to employ mask structures having smaller openings. Such smaller openings treat smaller portions of the substrate, thereby creating smaller structures in the photoresist. In order to produce smaller structures in the photoresist, shorter wavelength ultraviolet radiation is also used in conjunction with the mask to image the photoresist. Such shorter wavelengths of radiation have also been particularly effective at curing or hardening photoresist materials used in fabricating the devices.
Until recently, in forming electronic devices, photoresists have been used that are sensitive at g-line (436 nm) and i-line (365 mn) for most microelectronic applications. Examples photoresists sensitive to such wavelengths are novolak-type photoresists. As the desire to form smaller features, on the order of sub-micron and sub-half-micron, on substrates has increased, photoresists have been formulated that are sensitive to UV radiation in the range of about 248 nm. Such wavelengths are referred to as deep UV since they are deep within the UV range. Photoresists which are sensitive to such wavelengths are known as deep UV photoresists. Many deep UV photoresists differ significantly from commonly used photoresists used to make devices with larger sized features. In particular, deep UV photoresists differ from novolak-type formulations and also have different optical properties.
Typically, systems for creating electronic devices as described above include a UV radiation source for exposing the photoresist.
As methods for producing miniature electronic structures improve, the desire to produce even smaller structures has continued to increase. Problems encountered in further device miniaturization include obtaining desired resolution in the UV radiation source and improved focusing resolution and depth of focus of the UV radiation on the photoresist. Other problems encountered include radiation leakage through the mask. Radiation leakage has been addressed by ensuring that the UV radiation to which the mask and the photoresist are exposed is well within the deep UV range and, in particular, less than 245 nm in wavelength. Additionally, the problem of long exposure times to increase the penetration of the UV radiation through planarization layers. Further problems include the ability of the patterns formed in the resist to withstand high powered dry processes without the loss of the image integrity.
As stated above, progress in processes for forming structures in photoresists has led to the creation of sub-micron and even sub-half-micron structures. For example, structures as small as 0.3 microns have even been created. In addition to the above-described problems, another common problem encountered as structure size has decreased is that thinner layers of photoresist must be used to ensure, among other things, that depth of focus requirements of the exposure tool are met. The exposure tool referring to the radiation source, optics, mask and other components used to expose the photoresist. The photoresists used, especially at such lesser thicknesses are highly transmissive of ultraviolet wavelengths used. The transmissivity of the photoresist combined with the high reflectivity to the UV wavelengths of commonly used substrates results in the reflection of the UV radiation back into the photoresist resulting in further photochemical reactions taking place in the photoresist. The further photochemical reactions resulting from the UV radiation reflected off of the substrate typically result in uneven exposure of the photoresist.
As the light is reflected off of the substrate, standing waves may be created. As a result, the structure which was intended to be created by the mask will not be created, as particularly evidenced by inconsistent feature dimensions. This results in device error and possibly failure. For minimum feature sizes of greater than about 1.0 microns, the standing waves and dimensional instability can be minimized by a post-exposure bake process that may allow the photoresist to diffuse more evenly. However, the reduction in standing wave effects and dimensional stability produced by a post-exposure bake is insufficient for sub-half micron feature sizes.
In order to address the transmissivity and reflectivity problems, anti-reflective coatings have been developed which are applied to substrates prior to applying a photoresist. As the photoresist is exposed to UV radiation, the anti-reflective coating or ARC absorbs a substantial amount of the UV radiation. The ARC thereby prevents the radiation from reflecting off of the substrate and reacting with the parts of the photoresist which should not be reacted by the mask. Anti-reflective coatings greatly reduce the impact of highly reflective substrate surfaces as well as the impact of grainy substrate surfaces and topographical features on the substrate surface during deep UV imaging.
In lithography using G- and I-line UV wavelengths, inorganic and organic ARC films have been used. However, some organic ARC films have been found to be more effective with the photoresists that are sensitive at deep UV wavelengths. These organic ARC materials include polysulfones and polyureas.
Typical anti-reflective coatings can reduce the reflectance of a substrate from in the neighborhood of about 40% to about 50% up to nearly about 100%. Anti-reflective coatings have also improved focal depth for some film layers. One problem which is particularly aggravated by relatively transparent photoresist materials and highly reflective substrate surfaces is the creation of standing waves. By eliminating or at least greatly reducing problems associated with reflectivity, anti-reflective coatings allow for the formation of sub-micron structures in the photoresist, very faithful reproduction of the mask pattern, and nearly vertical resist edge profiles in the photoresist. Accordingly, it can be seen that anti-reflective coatings have been quite successful in helping to achieve the production of very small structures in photoresist on a substrate.
Although they have been quite useful to alleviate problems associated with reflectivity of substrate surfaces and transparency of photoresists, the use of anti-reflective coatings has created additional problems. These problems particularly manifest themselves, for example, after the deposition of the ARC and the photoresist layers, exposure of the photoresist, and development of the photoresist. After development of the photoresist, the remaining photoresist on the substrate may be exposed to ultraviolet radiation and elevated temperatures to cure or harden it. A process for hardening of photoresist is disclosed by U.S. patent application Ser. No. 497,688, filed May 23, 1983, now U.S. Pat. No. 4,548,688 to Matthews, the entire disclosure of which is hereby incorporated by reference.
Subsequent to development of the photoresist, the anti-reflective coating exposed by the developing of the photoresist must be removed prior to further processing the substrate. At this point, the additional problems become particularly apparent. For sub-micron work, the resist pattern must be transferred into the arc layer by dry etch processing. However, many problems are associated with ARC etching. For example, the ARC etches very slowly and at about a one to one selectivity with the commonly known photoresist. Therefore, the ARC must be subjected for an extended period to whatever process is used to remove it. Due to the low selectivity ratio, the photoresist may be removed as well.
The remaining photoresist on the substrate may be exposed to ultraviolet radiation and elevated temperatures to cure or harden it at this point, instead of or in addition to the above time, prior to removal of the exposed anti-reflective coating. The curing or hardening at this point may also be carried out according to the process described above, or any other known process.
As a result, critical dimension of the photoresist pattern can be altered. Additionally, overall resist thickness can be reduced before the substrate has even begun to be etched. Further, in the time required to remove the anti-reflective coating, the photoresist may be totally etched away, resulting in unintended exposure of the substrate. Such unintended exposure may result in defects in the device being produced.
The problems in etching the ARC layer are compounded by the necessity to completely etch the exposed ARC from the substrate prior to any further processing of the substrate so as to ensure proper processing of the substrate. Residues remaining from incomplete etching of the ARC typically cause problems after ARC etch. Among these problems are incomplete or inaccurate processing of the substrate as a result of anti-reflective coating residues not allowing the substrate to be treated.
One way in which the ARC etch problem has been addressed is by creating and manufacturing new equipment especially for removing anti-reflective coatings. However, such equipment is costly and additionally time and energy must be expended in design, construction and operation.
Another possible solution to the above-described problems associated with removal of the anti-reflective coating is to increase the thickness of the photoresist layer. Increasing the photoresist thickness could reduce the potential for unintended exposure of the substrate during processing. However, increasing the photoresist thickness does not solve the above problems because to create sub-micron structures in the resist, the resist must be very thin to meet the depth of focus requirements of the exposure tools used for patterning the photoresist. Increasing the thickness of the photoresist also increases the expense of producing devices through increased use of photoresist and increased waste of photoresist which is removed during ARC etching.
If the ARC etch problems are not dealt with, little resist may be left for the subsequent etching processes after the ARC has been etched.
It is an object of the present invention to provide a process for treating anti-reflective coatings.
Another object of the present invention is to provide a method of treating an anti-reflective coating which at least partially removes the anti-reflective coating.
An additional object of the present invention is to reduce the ARC etch time.
Also, an object of the present invention is to provide a method of forming an electronic device the method including treating an anti-reflective coating so as to remove the anti-reflective coating and/or change the time required for removal and/or improve the ARC to resist selectivity.
According to preferred aspects, the present invention provides a method of treating an anti-reflective coating on a substrate. The method includes exposing the anti-reflective coating to a dosage of ultraviolet energy sufficient to induce physical or photochemical reactions resulting in the removal of at least a portion of the exposed anti-reflective coating.
According to further preferred aspects of the invention, the invention provides a method of forming an electronic device. The method includes providing a substrate. A layer of an anti-reflective coating is deposited on the substrate. Then a layer of a photoresist material is deposited on the anti-reflective coating. Next, the photoresist is selectively exposed to ultraviolet radiation so as to selectively cause the photoresist to either polymerize or become more soluble, depending on type of resist. The photoresist is then developed with a developer to remove selected portions of the photoresist thereby selectively exposing portions of the anti-reflective coating underneath the portions of the photoresist removed by the developer. Subsequently, the exposed anti-reflective coating is subjected to a dosage of ultraviolet radiation sufficient to remove at least a portion of the exposed anti-reflective coating. A pattern of circuitry is then provided.